The calendar of Julius Caesar was a durable attempt to make the average length of the calendar year match the length of the tropical year. Its simplicity - add an extra day to February every four years - was its greatest virtue. By a stroke of luck, the monk Dionysius Exiguus calculated the year of the Nativity in such a way that leap-years Anno Domini are those which are divisible exactly by four, which is an easy rule to remember.
But this simplicity has a price. Four years in the Julian calendar are equal to 1461 days, so that the average length of the year is 365.25 days. This is 11 minutes 15 seconds longer than the true length of the tropical year. It may not seem very long - less than the time it takes to boil the kettle and make a cup of tea - but each year is too long by 11 minutes 15 seconds and the discrepancy builds up. After only 128 years, it becomes an entire day. Every 128 years, the seasons begin a day earlier in the calendar.
The ancient Egyptians lived quite happily with a calendar that allowed the seasons to slip by a day every four years. The Greeks and Romans were content to live with the haphazard intercalation required by a lunar calendar.
The Christian church, however, had fought bitter internal battles over the calendar, and especially over the date of Easter, which the Council of Nicaea had linked inextricably with the date of the Spring Equinox.
But the equinox was moving backward through the calendar! As early as the 8th century, the Venerable Bede had noticed that it no longer fell on March 21st, the day allotted to it by the Council of Nicaea. By the early Middle Ages, astronomers agreed that something must be done, but to change the calendar was not a step that could be taken lightly. Successive Popes studied the problem and declined to act.
It fell to Pope Gregory XIII to correct the accumulated error and to ensure that future generations would not face the same dilemma.
Pope Gregory XIII was born Ugo Buoncompagni in 1502 in Bologna. He studied law and became a lecturer and judge in his native town. In 1539 he went to Rome, and in 1549 he was sent to the Council of Trent, an ecumenical council which met fitfully over the course of some twenty years from 1545 to discuss matters of importance to the Roman Church. In 1565, Ugo was elected a cardinal and in 1572, became Pope, taking the name of Gregory XIII.
In 1577, Gregory sent a letter to all Catholic princes, describing his proposal for reforming the calendar. The letter was entitled Compendium novae rationis restituendi Kalendarium, or "Compendium of a New Way of Restoring the Calendar".
By 1582, aged 80, he was ready to act. He issued the apostolic letter Inter gravissimus which ensured his place alongside Julius Caesar as a man who could impose his will on the very course of time itself.
The name of the apostolic letter simply means "among the most serious" and is taken from the first sentence of the letter. In full, this reads:
Among the most serious tasks, last perhaps but not least of those which in our pastoral duty we must attend to, is to complete with the help of God what the Council of Trent has reserved to the Apostolic See.
The final session of the Council of Trent, in December 1563, had left it to the Pope to complete the reform of the Mass and the breviary. The latter also incorporated a provisional calendar reform, intended to correct the calendar's predictions of the dates of New Moon, which were by now four days out of step with the real Moon. New discrepancies were to be prevented by the inclusion of an additional leap day every 300 years, from 1800 onwards. Pope Gregory called together a commission to advise him on the reform of the calendar. One of its most assiduous members was Christopher Clavius. It was the commission's recommendations that the Pope adopted in Inter gravissimus.
The Gregorian reform of the calendar had three parts.
First, in order to restore the Spring Equinox to March 21st, the date set by the Council of Nicaea, ten days were to be omitted from the calendar in October 1582. Thursday October 4th was followed by Friday October 15th. The cycle of days of the week was not interrupted, but October 5th to 14th did not exist in the year 1582.
Second, in order to bring the average length of the calendar year into closer agreement with the length of the tropical year, three leap years were to be omitted in every four centuries. Every centurial year which was not divisible by 400 would not be a leap year.
This was a clever ploy. The next centurial year was 1600, only eighteen years away at the time of Inter Gravissimus, and it would be leap year in the new calendar as well as the old. Nobody living through Gregory's calendar reform would ever need to worry about the revised rule for leap years.
Nonetheless, it had the effect of making 400 years equivalent to 146097 days, giving an average calendar year of 365.2425 days, just 26.8 seconds longer than the tropical year. This discrepancy would amount to one day in 3200 years. No further reform of the calendar would be needed until the 49th Century A.D.!
Third, as the new leap year rule meant that the days of the week would no longer repeat every 28 years, the 532-year cycle of Victorius could no longer be used to construct tables of the dates of Easter. A new method for computing Easter had to be devised, and it required a set of arcane corrections to allow for the fact that ratio of the length of the calendar year to that of the lunar month had also changed. The dates of Easter in the new calendar would now repeat in a cycle which was 5,700,000 years long.